Add some more samples
make rect.js locally available
Add buffer object and update synths

* Buffer now supports duration
* Synths all now respond to :amp for amplitude changes
* All note-playing synths now respond to :attack and :release
* Samples now support :start and :end arguments (as vals between 0 and 1)
  to allow sub-parts of the sample to be played. To play the second half
  of a sample:  sample :foo, :start, 0.5. To play the first half use
  sample :foo, end, 0.5 etc.
Add utility function sample_duration, to return the duration of a sample
in seconds.
Add synonym Node#control for Node#ctl